Morphine withdrawal after prolonged drug treatment unmasks a paradoxical augmented sensitivity to painful (hyperalgesia) and innocuous (allodynia) stimuli. Activation of spinal microglia was shown to play an important role in the regulation of pain sensitivity. Interestingly, recently it was found that CB2-selective cannabinoid agonists efficiently antagonize glial activation. In addition, our preliminary shows that systemic co-administration of morphine with a CB2 agonist completely attenuates hyperalgesia and tactile allodynia upon morphine withdrawal in rats. Therefore, we intend to explore the hypothesis that co- administration of morphine with a CB2-selective agonist attenuates sustained morphine- mediated glial activation. The specific aims the proposed work are: I. to determine dose- response relationships for CB2 agonist-mediated attenuation of withdrawal hyperalgesia and allodynia in morphine treated rats;II. to determine the effect of sustained systemic co- administration of morphine and a CB2 agonist on spinal microglia and astrocyte activity and on spinal pain neurotransmitter (CGRP) concentration in rats;and III. to develop and characterize a primary neonatal rat spinal microglia culture as a future in vitro cellular model for a detailed investigation of the molecular mechanism of CB2 receptor-mediated microglial signaling. Since CB2 receptors are present in low concentrations in neurons, these agents exhibit little or no CNS side effects. Therefore, co-administration of opioid and CB2 agonists is expected to produce longer-lasting pain relief using lower morphine doses, reducing the likelihood of dangerous side effects in the treatment of chronic pain. PUBLIC HEALTH RELEVANCE: Prolonged morphine treatment leads to a gradual decline of pain relief. Consequently, clinicians need to use steadily increasing morphine doses to manage severe chronic pain (analgesic tolerance). Higher morphine doses on the other hand are more likely to cause serious side effects and may also lead to drug addiction. Interestingly, recently it was demonstrated that sustained exposure to morphine actually increases the sensitivity of patients to painful (hyperalgesia) and even to normally painless (allodynia) stimuli, contributing to the need for increased morphine doses in the treatment of chronic pain. Activation of spinal glia was shown to play a crucial role in the development of such paradoxical sensitization to sensory stimuli upon sustained morphine treatment. Accordingly, glial inhibitors were found to reduce both neuropathic pain sensitization and morphine tolerance. In the present proposal we intend to explore the utility of a novel pharmacological approach -activation of the CB2 cannabinoid receptor type - to inhibit spinal microglia activation during sustained morphine treatment. By preventing glia activation, opioid and CB2 agonist co-treatment is expected to produce more efficient pain relief without adaptive pain sensitization during sustained treatment. Thus, with such drug combinations, we may be able to achieve longer- lasting pain relief with lower morphine doses, reducing the likelihood of dangerous side effects and opiate addiction in the treatment of chronic pain.